The field of the disclosure relates generally to combined cycle fuel cell systems and, more particularly, to fuel cell carbon dioxide (CO2) removal systems and methods of operation thereof.
Fuel cells are electrochemical energy conversion devices that have demonstrated a potential for relatively high efficiency and low pollution in power generation. A fuel cell generally provides a direct current (DC) which may be converted to alternating current (AC) through, e.g., an inverter. The DC or AC voltage can be used to power motors, lights, and any number of electrical devices and systems. Fuel cells may operate in stationary, semi-stationary, or portable applications.
Certain fuel cells, such as solid oxide fuel cells (SOFCs), may operate in large-scale power systems that provide electricity to satisfy industrial and municipal needs. Others may be useful for smaller portable applications such as e.g., powering cars. Other common types of fuel cells include phosphoric acid (PAFC), molten carbonate (MCFC), and proton exchange membrane (PEMFC), all generally named after their electrolytes.
A fuel cell produces electricity by electrochemically combining a fuel and an oxidant across an ionic conducting layer. This ionic conducting layer, also labeled the electrolyte of the fuel cell, may be a liquid or solid. Fuel cells are typically amassed in electrical series in an assembly of fuel cells to produce power at useful voltages or currents. Therefore, interconnect structures may be used to connect or couple adjacent fuel cells in series or parallel. In general, components of a fuel cell include the electrolyte and two electrodes, i.e., an anode and a cathode. The reactions that produce electricity generally take place at the electrodes where a catalyst is typically disposed to speed the reactions. The electrodes may be constructed as, e.g., channels and porous layers to increase the surface area for the chemical reactions to occur. The electrolyte carries electrically charged particles from one electrode to the other and is otherwise substantially impermeable to both fuel and oxidant.
The opportunity for a power generation system that can benefit greatly from the integration of a fuel cell and a combustion apparatus derives in large part from the electrochemistry of the fuel cell. Typically, the fuel cell converts hydrogen (fuel) and oxygen (oxidant) into water (byproduct) to produce electricity. The byproduct water may exit the fuel cell as steam in high-temperature operations. This discharged steam (and other hot exhaust components) may be utilized in turbines and other applications to generate additional electricity or power, providing increased efficiency of power generation. If air is employed as the oxidant, the nitrogen in the air is substantially inert and typically passes through the fuel cell. Hydrogen fuel may be provided via local reforming (e.g., on-site steam reforming) of carbon-based feedstocks, such as reforming of the more readily available natural gas and other hydrocarbon fuels and feedstocks. Examples of hydrocarbon fuels include natural gas, methane, ethane, propane, methanol, syngas, and other hydrocarbons.
The reforming of hydrocarbon fuel to produce hydrogen to feed the electrochemical reaction may be incorporated with the operation of the fuel cell. Moreover, such reforming may occur internal and/or external to the fuel cell. For reforming of hydrocarbons performed external to the fuel cell, the associated external reformer may be positioned remote from or adjacent to the fuel cell. The reformed fuel may be channeled to the inlet of the anode to facilitate further supplying hydrogen fuel to the fuel cell. A portion of the reformed fuel may be channeled to a combustion engine for further electric power generation.
Fuel cell systems that can reform hydrocarbon internal and/or adjacent to the fuel cell may offer advantages, such as simplicity in design and operation. For example, the steam reforming reaction of hydrocarbons is typically endothermic, and therefore, internal reforming within the fuel cell or external reforming in an adjacent reformer may utilize the heat generated by the typically exothermic electrochemical reactions of the fuel cell. Furthermore, catalysts active in the electrochemical reaction of hydrogen and oxygen within the fuel cell to produce electricity may also facilitate internal reforming of hydrocarbon fuels. In SOFCs, for example, if nickel catalyst is disposed at an electrode, e.g., the anode to sustain the electrochemical reaction, the active nickel catalyst may also reform hydrocarbon fuel into hydrogen and carbon monoxide (CO). Moreover, both hydrogen and CO may be produced when reforming hydrocarbon feedstock. Thus, fuel cells, such as SOFCs, that can utilize CO as fuel (in addition to hydrogen) are generally more attractive candidates for utilizing reformed hydrocarbon and for internal and/or adjacent reforming of hydrocarbon fuel.
As described previously, the exhaust components from fuel cells that operate at high temperatures can be directed to turbines and other types of engines, as part of a general combined cycle system. However, some known combined cycle systems that include fuel cells facilitate energy losses in the form of heat losses such that fuel cells routinely achieve a conversion efficiency that is only about 50%.
In addition to hydrogen and CO, internal fuel cell reforming may generate carbon dioxide (CO2) that is entrained in the reformed fuel stream. As the combined cycle system is scaled upward, generating sufficient reformed fuel to fire a combustion engine will increase the amount of CO2 generated. Excess CO2 in the reformed fuel stream may adversely affect operation of the fuel cells and combustion engines.